In addition to detecting position changes of two objects that are movable with respect to each other in one or two lateral directions, there are measuring tasks in which it is exclusively or possibly additionally necessary to determine the distance between these objects in a vertical measuring direction that is perpendicular thereto. Interferential methods lend themselves to a highly precise distance measurement along such a measuring direction, as disclosed, for example, in German Published Patent Application No. 10 2007 016 774 or German Published Patent Application No. 10 2011 005 937.
In these interferential distance measurements, a beam of rays is split into two partial beams via suitable diffractive or refractive optical elements, that is, into a measuring beam and a reference beam. Subsequently these pass through associated measurement and reference arms and are brought into an interfering superimposition at a merging location. The distance to be measured is encoded via the phase difference between the measuring beam and the reference beam. In the event that there exists a difference of path length between the measuring beam and the reference beam, there results a dependence of the distance to be measured on the respective wave length of the individual beams of rays. Fundamentally, however, independence is desired of the interferential distance measurement from possible wave length fluctuations. In the methods from the two above-mentioned documents, however, this is only ensured at a specific nominal distance, but not over the entire distance measuring range.